This invention relates to a method and apparatus for separating fluids in a drilling fluids system and more particularly separating water and oil phases of the fluids wherein oil is produced from earth formations into the drilling fluids.
In the drilling of horizontal boreholes into earth formations, problems different than those in conventional drilling operations are encountered with respect to drilling the formations, and particularly with respect to the drilling fluids system. One purpose for drilling horizontally into a formation is to increase the surface area of the producing formation which is exposed to a production flowpath. In a standard drilling operation the drill pipe passes through the formation to be produced in a substantially perpendicular fashion. Thus, the surface area of the formation that is exposed to the production pipe is typically the thickness of the formation. Where the completion is made horizontally through the formation, the surface area of the production zone is only limited by the pratical length at which drilling can be extended horizontally into a formation. In addition, such wells are sometimes drilled into consolidated formations wherein casing is not used in the completion, thus the production is by way of an open hole into a production pipe string. This being the case, any hydrocarbon fluids in the formation being drilled are of course exposed to the wellbore that is penetrating through the formation. In order to prevent or minimize formation damage as the drilling operation is taking place, it is preferred not to use a drilling mud containing materials that will penetrate into the formation or cake over the formation to thereby limit production from the formation during or after the drilling operation. Typically, drilling fluids contain granular solid material that when mixed with water forms a clay cake on the formation face penetrated by the wellbore. In the typical drilling operation, casing is cemented into the hole over this caked face and then a perforating operation is performed which opens holes through the casing, cement, cake face, and into the formation to expose formation to the wellbore and thereby provide a communication path for formation fluids into the wellbore to produce the well. In horizontal drilling operations a water or brine solution is sometimes used as a drilling fluid to prevent damage to the formation. These drilling fluids are generally of a lower density than the typical drilling muds and at the same time do not form a substantial filter cake on the wellbore. The lower density of the fluid produces a pressure underbalance so that any hydrocarbon fluids in the formation, particularly those under pressure, will produce into the wellbore and become entrained in or mixed with the drilling fluids therein. It has been found that it is possible to produce these formation fluids into the drilling fluids and recover them at the surface for subsequent sale of the produced hydrocarbon fluids. If a substantial amount of such hydrocarbon fluids are produced during the drilling operation, this has the possibility of being a favorable economic situation, thus making it desirable to produce the hydrocarbon fluids and to provide a separation process at the surface which will permit the sale of such produced fluids.
In the typical drilling fluids system used in vertical wells we refer to the drilling fluid system as being a balanced system when the weight of the drilling fluids is such that it prevents substantial intrusion of formation fluids into the wellbore as it is being drilled i.e, the drilling fluid density, hydrostatic pressure and pump pressure are balanced as a system with the formation pressure. If the pressure rises in the formation the density of the drilling fluids is increased to compensate for this rise in pressure and thereby maintain the balance. In a well, such as a horizontally drilled well, where fluids are purposefully being produced, the drilling fluid system is run in an underbalance condition to allow the formation fluids to flow into the wellbore. This is sometimes called "going live" in drilling jargon. The choke manifold at the surface is opened to permit pressure on the formation to be reduced which in turn lets the formation flow. Thus in horizontal drilling under the above circumstances, we are interested in minimizing caking or damage to the wellbore, producing fluids during drilling, recovering as much of the produced fluids as possible to provide a revenue during the drilling operation period, and maintaining control of the density of the drilling fluids so that the drilling fluid density is sufficiently low to permit production and at the same time heavy enough to maintain well control as to prevent the formation pressure from caving in the borehole blowing out the well.
The problems presented by this present drilling system, with respect to producing hydrocarbons during the drilling operation, have to do in one important respect with the gravity separation methods that are presently used to separate oil and water. An extensive amount of tankage is required to accommodate the drilling fluids and produced hydrocarbons in order to provide sufficient separation to recover clean production fluids and at the same time remove a sufficient amount of the hydrocarbons from the drilling fluid to maintain its density at a usable level. The hydrocarbons are normally less dense than the drilling fluids and therefore if very much hydrocarbon fluid is left in the drilling fluids after separation and before recirculation, the density of the drilling fluids is adversely affected. Therefore, the efficiency of oil removal from the circulated drilling fluids is a critical issue in these systems using the present gravitational separation methods. Another factor that is addressed with the present invention in addition to separation efficiency and economics of producing hydrocarbons during drilling, is that of safety. One safety issue is the control over drilling fluid density which has to do with the prevention of blowouts and wellbore cave-in. In these systems, the system pressure is borne by portions of the separation system. When you are drilling with an unbalanced system "gone live", the control of fluid density versus formation pressures becomes more cirtical and accurate control of hydrocarbon fluids in the drilling fluids is very important. More careful control of fluid density can minimize the complexity of process control within the system. In additon, in a gravity oriented system, there is often a number of residence tanks vented to the atomsphere which are holding fluids containing hydrocarbons. One of the more compelling factors for improving the prior separation systems is the capitol costs and logistics involved in the extensive tankage, lines, controls, etc. involved in the gravity system in addition to separation efficiency and the inherent safety improvements resulting from a simpler system.
It is therefore an object of the present invention to provide a simpler, more efficient, and less costly method and apparatus for dealing with the problem of separating drilling fluids from production fluids in a well being drilled into a producing formation. In addition, it is an object to provide a safer system and one which will reduce costly rig up and rig down time when the drilling location is changed. When such horizontal drilling operations are conducted offshore, it becomes a further object to provide simplified systems from a space saving standpoint which also meet the enhanced requirements for safety in offshore operations.